The present invention pertains to a fuel cell system in which a fuel such as methanol is fed in liquid form to the fuel cells in the so-called DMFC (Direct Methanol Fuel Cell) system with a container holding a supply of fuel, a conduit designed to supply liquid fuel which leads from the container to the fuel cells, another conduit which leads back to the container from the fuel cells, a nozzle provided in the other conduit and a pressure-boosting pump present in the conduit system.
A fuel cell system of this type is known from DE 197 45 773 A1.
As described in that document, there are various types of fuel cells, including so-called SOFC fuel cells which work at operating temperatures above 1000xc2x0 C. and so-called PEM fuel cells which have an operating temperature of about 80xc2x0 C.
Systems are also known in which a liquid fuel such as methanol can be oxidized on the anode of a PEM fuel cell by means of a catalyst such as platinum, resulting in the release of hydrogen.
The present invention concerns such fuel cells in which fuel is supplied in liquid form. As the fuel, methanol above all, but also other hydrocarbons, such as hydrazine, come into consideration. Mixtures of hydrocarbons and water can also be used as liquid fuels. The liquid fuel is fed to the fuel cells on the anode side.
When methanol is used as the fuel, the liquid fed to the fuel cells usually consists of 3% CH3OH and 97% H2O. By reaction with water, part of the CH3OH is transformed into CO2 so that the mixture leaving the fuel cell consists of H2O, CH3OH and CO2: 
The protons thus generated diffuse through the membrane toward the cathode side of the fuel cell, while the electrons pass to the cathode side via the external current circuit. The protons and electrons combine with the oxygen supplied to the cathode side according to the equation: 
and thus form water. This water is usually fed into the fuel, e.g., fed back since water is required for the reaction of equation 1.
The fuel is frequently supplied supersoichiometrically to the fuel cells so that it is not completely reacted. Liquid fuel therefore emerges from the fuel cells.
In such a DMFC System, the CO2 accumulating as product on the anode side must be sluiced out of the anode cycle. In addition to gas bubbles in the liquid which are relatively easily removed, a certain quantity of CO2 is also dissolved in the liquid. The presence of the reaction product CO2, partly in dissolved form and partly as gas bubbles, in the fuel cell unfavorably influences the power output of the fuel cell. For this reason, it has been attempted, according to the above-cited document DE 197 45 773 A1, to remove the CO2 from the cycle by raising the pressure of the mixture leaving the fuel cells and subsequently depressurizing it for the purpose of causing the gas dissolved in the liquid to pass over into the gas phase upon depressurization, thus making it easily separated from the liquid components.
The arrangement according to De 197 45 773 A1, however, has a disadvantage since the increase in pressure and the subsequent depressurization is arranged in the conduit line leading from fuel cells to the container. Although it is correct that the CO2 appearing at the nozzle upon depressurization escapes, the quantity is limited in principle to that quantity which had passed into solution additionally at the pump arranged in front of the nozzle due to the increase in pressure. The mixture downstream from the nozzle still always contains dissolved CO2 and other gases. An equilibrium is established in the container according to which the mixture departing the container is saturated with CO2 corresponding to the pressure prevailing there and the temperature prevailing there. The saturated mixture is then fed to the fuel cell.
The purpose of the present invention is to improve the system described initially in such a way that the liquid supplied to the fuel cells is undersaturated with CO2.
To solve this problem, the invention provides that the pump be arranged in the conduit line leading from the container to the fuel cells. However, the nozzle should remain at the previously chosen position in the other conduit line.
From the physical aspect, the higher the temperature and the lower the pressure, the less CO2 is dissolved in the liquid. The arrangement according to the present invention has the advantage that the mixture leaving the container is saturated with CO2, is brought to a higher pressure level by the pump and is therefore automatically undersaturated. Therefore the mixture in the fuel cell can absorb gaseous CO2 in a quantity corresponding to the higher pressure. At the nozzle, the mixture is depressurized again and the CO2 dissolved in the liquid passes over into the gas phase. Therefore, part of the dissolved CO2 can be separated out according to the invention and removed from the system so that the liquid reaching the fuel cells is actually undersaturated.
This is advantageous for the operation of the fuel cells, because part of the reaction product CO2 can now be dissolved in the undersaturated liquid and need not be discharged as a gas.
According to a preferred variant of the invention, a cooler is arranged in the conduit line leading from the container to the fuel cells, said cooler preferably being arranged in front of the pump, but it may also be arranged after the pump.
This variant allows for the fact that in the case of lower temperatures more CO2 passes into solution so that by cooling the liquid entering into the fuel cells it can absorb more CO2.
While in the state of the art one always strives to separate CO2 from the system, only with the invention does one succeed in feeding a liquid undersaturated with CO2 to the fuel cells so that the liquid is actually capable of absorbing CO2 into the fuel cells and of clearly reducing the quantity of gaseous CO2.
Especially preferred variants of the invention are presented in the other claims.